The present invention relates to a distance measurement apparatus for automatically performing focus control of a camera in accordance with the distance to an object to be photographed. More particularly, the present invention relates to a method that can cope when distance measurement is impossible in a distance measurement apparatus for detecting object distance by obtaining the relative image distance between the first and second object images formed by a pair of lenses spaced apart at a predetermined interval.
In a conventional camera using a passive AF (Auto Focus) function, when distance measurement is impossible, the shutter is locked (kept inoperative) or the focusing lens is extended to a predetermined position regardless of the object distance, and then the shutter is operated (actuated).
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-84231, there is also known a method of extending the focusing lens to infinity for a bright object not requiring use of an electronic flash, and to the position to which light from the electronic flash can reach for a dark object requiring use of the electronic flash.
Jpn. Pat. Appln. KOKAI Publication Nos. 64-78205 and 64-78206 disclose techniques for measuring the distance to a low-luminance, low-contrast object by phase difference detection using CCD output data of only auxiliary light.
The conventional techniques except lock processing in all the prior arts described above extend the focusing lens regardless of the object distance and photograph, thereby resulting in a large number of out-of-focus photographs. These prior arts were primarily devised to eliminate the confusion and dissatisfaction experienced when the user cannot take photographs. A large number of out-of-focus photographs, however, do not mean that this objective has satisfactorily been achieved. How to reduce this large number of out-of-focus photographs is still left unsolved.
The techniques disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 64-78205 and 64-78206 take a long time to acquire (integrate) data enough to operate correlation for CCD sensor elements in the ON state of auxiliary light. In addition, the complexity of distance measurement operation requires a high-speed arithmetic means (CPU), resulting in a high-cost apparatus.